Containers used in the cosmetics, food and drug industries have in the past generally been constructed of glass or certain plastic resins, e.g., polyvinyl chloride, polypropylene, polycarbonate or polyacrylonitrile resins. In many instances, these containers have proven to be unsatisfactory or undesirable from the standpoint of cost or biodegradability or for various other reasons. For example, it has been discovered that containers, e.g., bottles, constructed of polyacrylonitrile resin give off noxious fumes when incinerated, causing severe environmental pollution problems. Recently, the use of polyacrylonitrile resin in such bottle construction has been stopped completely. Other containers, such as glass, have obvious drawbacks, being readily breakable, bulky to transport and expensive to manufacture, requiring excessive energy consumption.
One material which has been investigated as a possible source of improved construction material for containers in the cosmetics, food and drug industries has been polyethylene terephthalate (PET). However, because of the fluid melt characteristics of PET, new, expensive and specialized equipment has had to be developed to process PET into containers and bottles. This equipment is generally based on processes which involve injection molding of a cylindrical or tubular preform which is subsequently reheated and simultaneously stretched and blown in another machine to form a container or bottle. See, e.g., U.S. Pat. No. 3,778,214.
Attempts to modify PET for use in injection molding processes include, e.g., U.S. Pat. Nos. 3,692,744 and 3,673,139. These patents deal with the incorporation of a chain branching agent, alone or in combination with a crystallization-promoting agent into PET in an effort to produce a composition which may be injection molded. The drawbacks of such injection molding processes have been already pointed out. While the later of these patents suggests that PET modified with a chain branching agent and a crystallization promoting agent may be suitable for extrusion molding applications, such suitability is expressly limited to the production of semi-final goods such as rods, sheets and embossed sheets. The process is not disclosed as applicable to extrusion blow molding of a parison to form hollow structures such as containers or bottles. In fact, it had not been possible until the present invention to successfully mold containers and bottles of PET which would serve as acceptable cosmetics, food or drug containers by the use of conventional extrusion blow molding equipment common in these industries.
In the past, in other areas of application unrelated to the extrusion blow molding of containers and bottles, attempts have been made to modify various polyesters, some including PET, by the incorporation of a chain branching agent or a chain terminating agent or both.
The attempts have been concerned with rendering polyesters suitable for use, e.g., as electrical or thermal insulating materials, coating compositions, or fiber and filament-forming materials with increased dye receptivity. Representative examples of such attempts include U.S. Pat. Nos. 2,606,161; 2,936,296; 2,951,827; 3,033,824; 3,039,979; 3,055,867; 3,223,666; and 3,546,180.
It was not until the present invention, however, in the course of a continuing search for acceptable alternatives to conventional bottle construction materials, that it was discovered that a high melt-strength, gel-free polyethylene terephthalate suitable for use in conventional extrusion blow molding applications could be obtained by the incorporation of specific amounts of selected chain branching and chain terminating agents in combination.